This post marks the official opening of the mini-polymath4 project to solve a problem from the 2012 IMO. This time, I have selected Q3, which has an interesting game-theoretic flavour to it.

Problem 3.Theliar’s guessing gameis a game played between two players and . The rules of the game depend on two positive integers and which are known to both players.At the start of the game, chooses two integers and with . Player keeps secret, and truthfully tells to player . Player now tries to obtain information about by asking player A questions as follows. Each question consists of specifying an arbitrary set of positive integers (possibly one specified in a previous question), and asking whether belongs to . Player may ask as many such questions as he wishes. After each question, player must immediately answer it with

yesorno, but is allowed to lie as many times as she wishes; the only restriction is that, among any consecutive answers, at least one answer must be truthful.After has asked as many questions as he wants, he must specify a set of at most positive integers. If belongs to , then wins; otherwise, he loses. Prove that:

- If , then can guarantee a win.
- For all sufficiently large , there exists an integer such that cannot guarantee a win.

**All are welcome.**Everyone (regardless of mathematical level) is welcome to participate. Even very simple or “obvious” comments, or comments that help clarify a previous observation, can be valuable.**No spoilers!**It is inevitable that solutions to this problem will become available on the internet very shortly. If you are intending to participate in this project, I ask that you refrain from looking up these solutions, and that those of you have already seen a solution to the problem refrain from giving out spoilers, until at least one solution has already been obtained organically from the project.**Not a race.**This is**not**intended to be a race between individuals; the purpose of the polymath experiment is to solve problems*collaboratively*rather than individually, by proceeding via a multitude of small observations and steps shared between all participants. If you find yourself tempted to work out the entire problem by yourself in isolation, I would request that you refrain from revealing any solutions you obtain in this manner until*after*the main project has reached at least one solution on its own.**Update the wiki.**Once the number of comments here becomes too large to easily digest at once, participants are encouraged to work on the wiki page to summarise the progress made so far, to help others get up to speed on the status of the project.**Metacomments go in the discussion thread.**Any non-research discussions regarding the project (e.g. organisational suggestions, or commentary on the current progress) should be made at the discussion thread.**Be polite and constructive, and make your comments as easy to understand as possible.**Bear in mind that the mathematical level and background of participants may vary widely.

Have fun!